The present invention relates to hermetic electronics packages constructed from a primary metallic material and having one or more secondary regions composed of material having disparate physical and/or structural properties. More specifically, electronics packages are provided with one or more regions of a secondary material having a high thermal conductivity and a coefficient of thermal expansion (CTE) that generally matches the CTE of the primary metallic material. In one embodiment, the electronics package comprises a primary metallic titanium component metallurgically bonded to a secondary metal matrix material, such as aluminum silicon carbide.
Electronic components are used in countless applications in a wide variety of environments. Such components are subject to faulty operation, degradation and corrosion resulting from contact with dust, water vapor, gases, and the like, as well as from high IC) temperature and/or pressure conditions. Such components are, therefore, generally sealed in a hermetic electronics package to provide protection from the operating environment.
Electronics packages typically comprise a box-like structure, in the interior of which the electronic components are mounted. The package is generally provided with feedthrough holes through which conductive wires are sealed with an insulator such as glass or ceramic and are used to operatively connect electronic components inside the package to electronic and electrical sources and systems outside the package. The conductive wires are electrically insulated from and hermetically sealed into the package. After the electronic components are mounted in the package and operatively connected to the conductive wires, a cover is hermetically sealed to the package base to seal the interior of the electronics package.
Electronics packages are desirably constructed from materials that meet application specific requirements for density, thermal expansion, thermal conductivity, mechanical strength, and the like. For example, electronics packages used in aircraft and spacecraft applications must be lightweight and are therefore constructed from low density materials. Electronics packages that are used in high power applications should be constructed from materials having a high thermal conductivity, so that heat generated within the package is conducted outside the package to maintain lower operating temperature conditions inside the package. In other words, heat must be efficiently dissipated from inside the package. The service life of components is increased considerably when lower temperatures are maintained within the electronics package.
Electronics packages are desirably constructed from materials having a coefficient of thermal expansion (CTE) approximately equal to that of the materials the packages contact. That is, the thermal expansion properties of the electronics package must be compatible with the thermal expansion properties of the circuitry mounted in the package. Otherwise, temperature changes produce stress between the package and its electrical circuitry as they expand and contract at different rates. Additionally, because electronic components are often mounted on ceramic chips having a low CTE, most electronic packaging applications require package materials having a low CTE, generally matching or slightly higher than that of the ceramic chips. The ceramic chips comprise materials such as silicon, gallium-arsenide and alumina. These materials are fragile and susceptible to breakage if they are mounted to a package having an incompatible CTE. Therefore, it is important to mount these chips to a package with similar expansion rate characteristics.
Electronics packages constructed from ferrous alloys, such as Alloy 52 or KOVAR, have a desirably low CTE but are relatively heavy. Electronics packages constructed from aluminum are light in weight, but the CTE of aluminum is higher than desirable and is incompatible with the thermal expansion rates of conventional ceramic chips. Furthermore, many of these electronic chips generate significant heat when operating and it is important for these chips to be mounted to a material having high thermal conductivity characteristics that will dissipate the heat generated by the chips.
Thus, materials having a low density (for light weight applications), a CTE matching or slightly higher than the CTE of the electronic circuitry, high thermal conductivity for heat dissipation, good mechanical strength, and the ability to be hermetically sealed during final package assembly are desirable for electronics packaging applications.
Historically, hermetic electronic packages have been fabricated using a variety of techniques. One conventional approach involves machining or otherwise fabricating the entire package from an iron-based metal such as KOVAR, alloy 42 or a ferrous metal having similar properties. The feedthroughs and/or connectors are installed with standard glass-to-metal-seal technology, or are soldered, brazed or welded directly into the package. These packages have the advantage that the package and the electronic chips have compatible CTE""s. They are disadvantageous, however, in that the package is heavy and has poor thermal conductivity. These types of packages are effectively limited to housing circuitry for non-power devices for applications other than aerospace.
Alternatively, the walls of the package may be manufactured from an iron-based alloy such as KOVAR, with the floor of the package composed of a composite metal or metal matrix material having a compatible CTE to that of the metallic package walls. The walls may be joined to the floor to form the electronics package using soldering or brazing techniques. Soldering requires plating of both the wall and floor sections. Welding techniques cannot be is used as a consequence of material incompatibility. Exemplary package floor materials include composite metals such as copper/molybdenum, copper/tungsten, and beryllium/aluminum, and metal matrix materials such as aluminum silicon-carbide, aluminum aluminum-oxide, copper graphite, and beryllium beryllium-oxide.
Another approach involves machining the entire electronics package from an aluminum alloy, such as alloy 6061. The feedthroughs and/or connectors are installed into the package by means of soldering or welding. An aluminum cover may be hermetically sealed to the package base using standard laser welding techniques. This package has the advantage of being lightweight and having a relatively high thermal conductivity, but the CTE of aluminum and aluminum alloys is generally incompatible with the CTE of the electronic chips. An alternative approach involves machining the entire electronics package base and/or connectors from a composite metal such as A40, an aluminum silicon composite, or ALBEMET(trademark), a beryllium/aluminum metallic composite. The feedthroughs and/or connectors may be installed by means of specialized welding or plating soldering techniques.
Metal matrix composite materials, which incorporate a non-metallic reinforcing material dispersed within a metal matrix or host material, generally have desirable properties for electronics packaging applications, including a low density, low CTE, high thermal conductivity and good mechanical strength. These properties may be manipulated somewhat by selecting the metal matrix material and the form, proportion and composition of the reinforcing material. Metal matrix materials comprising aluminum or aluminum alloy matrices incorporating silicon carbide reinforcement material have low density, low CTE, good thermal conductivity, and suitable mechanical strength for use in electronics packaging applications.
Although metal matrix composite materials have desirable properties for use in electronics packaging applications, they have several practical disadvantages. Metal matrix materials cannot be machined using conventional tools and are provided in a three-dimensional configuration, such as an electronics package, using a casting process. The use of casting techniques limits the tolerances and versatility of electronics packages and increases the cost of producing the packages. Metal matrix materials generally cannot be laser welded as a consequence of differences in energy absorption rates between the metal matrix material and the non-metal reinforcing material. Feedthroughs and connectors may be sealed in metal matrix composite materials using soldering or welding techniques such as those described in U.S. Pat. No. 5,526,867.
One methodology for hermetically sealing a cover to an aluminum metal matrix electronics package is to fabricate a package base in which the upper sidewall rims are free of the reinforcing material, as described in xe2x80x9cInvestment Cast Metal Matrix Composite,xe2x80x9d by S. Kennerknecht, Society of Manufacturing Engineers Technical Paper EM90-411, 1990, available from the Society of Manufacturing Engineers in Dearborn, Mich. An aluminum/silicon carbide base may be cast using a preform, for example, forming an aluminum upper rim of the base sidewalls that is substantially free of the reinforcing material. A package cover, comprising aluminum or an aluminum alloy, may then be laser welded to the aluminum rim. Electronics packages having a metallic rim welded to an aluminum cover tend to warp, however, upon thermal cycling as a consequence of the difference between the CTE of the metal matrix composite package material and the CTE of the metallic rim and cover. This arrangement is generally useful only for very small electronics package applications in which warpage during operation is minor, but it is not suitable for larger electronics packages that are subjected to thermal cycling during operation.
Electronic packages may also be manufactured from metal matrix composite materials by means of infiltrating ceramic preforms with molten metal. This can be done by vacuum/pressure infiltration as described in U.S. Pat. No. 5,406,029, or by spontaneous infiltration as described in U.S. Pat. No. 5,526,867. The metal matrix composite package base is CTE compatible, has good thermal conductivity, and is lightweight. A three-dimensional metal matrix composite package base is not weldable, however, without the addition of metal inserts at the weld areas. Manufacturing techniques for producing metal matrix composite electronics bases generally are not cost competitive as a consequence of mold tooling and process complexity.
The present invention provides electronics packages constructed from a primary metal material metallurgically bonded to one or more secondary regions having desirable thermal conductivity properties and having a CTE that generally matches the CTE of the primary metallic package material. The secondary regions serve as heat sinks and provide for thermal dissipation from inside the electronics package or for thermal dissipation from a point source over a larger surface area. The primary metallic package material is preferably chemically reactive with a metallic constituent of the secondary region, providing a metallurgical bond between the primary metallic package material and the one or more secondary regions. The CTE match of the secondary region with the primary metallic package and metallurgical bond formed between the secondary region and the primary metallic package material provide an electronics package having reliable hermeticity and desirable mechanical strength properties.
One or more thermal coefficient of expansion property modifiers, such as one or more of the non-metallic or metallic reinforcing material(s), may be incorporated into the secondary material in a proportion sufficient to modify the CTE of the secondary region so that it is compatible with the CTE of the primary metallic package material. The metallic constituent of the secondary material preferably has a higher thermal conductivity than that of the primary metallic package material. Thus, the thermal conductivity of the secondary region is different from (generally higher than) the thermal conductivity of the primary metallic electronics package material, yet the CTE of the secondary region substantially matches the CTE of the primary metallic package material. According to preferred embodiments, the CTE of the secondary region also substantially matches or is slightly higher than the CTE of electronics components or substrates for such electronics components, such as ceramic chips, that are mountable in the interior of the electronics package.
According to one embodiment, the primary metallic electronics package material comprises titanium, or a titanium alloy, and the one or more secondary regions(s) comprises a metallic or metal matrix composite material having a high thermal conductivity, such as aluminum with tungsten or molybdenum, or aluminum silicon carbide (AlSiC). In this embodiment, the metallic aluminum component of the secondary material bonds metallurgically with the titanium component of the primary metallic package at the interface of the composite region(s) with the metallic package base. A desired proportion of a reinforcing material such as tungsten or molybdenum or silicon carbide material may be provided in the secondary material such that the secondary region has a CTE that substantially matches the CTE of the surrounding metallic electronics package material and is compatible with the CTE of ceramic circuitry. According to one embodiment, the CTE of the secondary region is slightly less than the CTE of the surrounding primary metallic electronics package material.
According to another embodiment, the primary metallic electronics package material comprises titanium or a titanium alloy, and one or more secondary regions comprises a secondary material having a high thermal conductivity, such as a composite metallic material. The term xe2x80x9ccompositexe2x80x9d material, as used herein, refers to compositions composed of multiple base materials that are present in a variety of chemical and physical configurations. Composite materials thus comprehend both conventional xe2x80x9ccompositexe2x80x9d materials, in which the constituents retain their individual chemical and structural integrity, and alloys, in which the constituents combine to form entities distinct from the individual constituents. Using this combination of primary and secondary materials, a metallurgical bond may be formed between the metallic package material and the secondary composite material. Additionally, the secondary metallic composite material has a high thermal conductivity compared to the thermal conductivity of the primary metallic material, yet the CTE of the secondary composite material substantially matches the CTE of the primary metallic package material.
The electronics package of the present invention has desirably lightweight and low density properties that are particularly suitable for aircraft and spacecraft applications. The primary metallic package may be economically and conveniently constructed using numerous conventional techniques, including machining, metal injection molding, casting, forging, or superplastic forming techniques. Using an electronics package material that is machinable is desirable because machining operations may achieve high tolerances and also provide versatility, since modifications to the three dimensional configuration of the electronics package base may be provided simply by modifying the machining process.
Feedthroughs, connectors, covers and other devices are preferably compatible with the primary metallic electronics package material and may thus be hermetically sealed to the metallic electronics package directly using conventional fusion welding techniques, such as laser or E-beam welding. If the electronics package base material comprises titanium, the feedthrough pins, connectors, accessories and package cover preferably comprise titanium or a material having thermal properties compatible with those of titanium. Alternatively, methods and systems for hermetically sealing metallic materials having incompatible thermal properties are described, for example, in U.S. Pat. Nos. 5,298,683, 5,433,260, 5,675,122, 5,110,307, 5,405,272, 5,041,019, 5,109,594 and 4,690,480. Any of these techniques may be used to hermetically seal feedthrough pins, connectors, accessories and package covers to a metallic electronics package base of the present invention.